An example of the heat-transfer unit used in a prior art heat exchanger is shown in FIG. 12.
The drawing is a partial perspective view of the conventional heat-transfer unit that is generally indicated by (1) and disposed in the direction of the flow of a fluid (A) (as indicated by the arrows). The heat-transfer element (1) is basically composed of heat-transfer fins, a heat generator, a heat absorber, a heat accumulator, and a heat radiator. In FIG. 6, the heat-transfer unit consists of a plurality of heat-transfer elements (1a), (1b) and (1c) that are stacked one on top of another and the fluid flows through the passage formed by adjacent heat-transfer elements. Each heat-transfer element (1) is cyclically bent in the direction of fluid flow in the form of trapezoidal waves, the bends in one element being in phase with those in an adjacent element.
The heat-transfer unit of the type described above is hereinafter referred to as an imperforate trapezoidally corrugated plate.
FIG. 13 is a partial perspective view of another conventional heat-transfer unit that consists of a plurality of heat-transfer elements (1) in a plane plate form that are disposed in the direction of the flow of a fluid (A) (as indicated by the arrows). This type of heat-transfer unit is hereinafter referred to as parallel plates.
FIG. 2 is a graph showing the heat-transfer characteristics of the two conventional types of heat-transfer unit, in which the characteristics of the imperforate trapezoidally corrugated plate are indicated by .circle. and those of the parallel plates by . The symbols on the x- and y-axes of the graph are: EQU Re=v.multidot.De/.nu.: Reynolds number; EQU Nu=h/De/.lambda.: Nusselt number
where
v: maximum velocity of wind passing through the heat-transfer unit; PA1 De: spacing between heat-transfer surfaces multiplified by a factor of 2; PA1 h: heat transfer rate; PA1 .nu.: fluid dynamic viscosity coefficient; and PA1 .lambda.: fluid heat conductivity.
As is clear from FIG. 2, the imperforate trapezoidally corrugate plate type heat-transfer unit shown in FIG. 12 and the parallel-plate type heat-transfer unit shown in FIG. 13 have essentially the same heat-transfer characteristics. In the heat-transfer unit of the type shown in FIG. 12, the fluid flows along the individual heat-transfer elements and this would provide the unit with heat-transfer characteristics which are essentially the same as those exhibited by the parallel-plate type heat-transfer unit.